RT5712B - Richtek

RT5712B
2A, 1MHz, 5.5V CMCOT Synchronous Step-Down Converter
General Description
Features
The RT5712B is a high efficiency synchronous
step-down DC/DC converter. Its input voltage range is
from 2.7V to 5.5V and provides an adjustable regulated
output voltage from 0.6V to 3.4V while delivering up to
2A of output current.








compensation allows the transient response to be
optimized over a wide range of loads and output
capacitors.



STB, Cable Modem, & xDSL Platforms
LCD TV Power Supply & Metering Platforms

General Purpose Point of Load (POL)
Fixed Soft-Start 1.2ms
Cycle-by-Cycle Over Current Protection
Input Under Voltage Lockout
Output Under Voltage Protection (UVP Hiccup)
Thermal Shutdown Protection
RT512BHGQW
3K : Product Code
W : Date Code
3KW
Ordering Information
RT512BLGQW
3J : Product Code
W : Date Code
RT5712B
3JW
Package Type
QW : WDFN-6L 2x2 (W-Type)
Lead Plating System
G : Green (Halogen Free and Pb Free)
Pin Configurations
UVP Option
H : Hiccup
L : Latch-Off
IC
1
EN
VIN
2
GND
(TOP VIEW)
Note :
Richtek products are :
3
7
6
FB
5
GND
LX
4
RoHS compliant and compatible with the current
WDFN-6L 2x2
requirements of IPC/JEDEC J-STD-020.

Best
Marking Information
Applications

for
Transient Response, Robust Loop Stability with
Low-ESR (MLCC) COUT
The internal synchronous low on-resistance power
switches increase efficiency and eliminate the need for
an external Schottky diode. The Current Mode
Constant-On-time (CMCOT) operation with internal

Efficiency Up to 95%
RDSON 100m HS / 70m LS
VIN Range 2.7V to 5.5V
VREF 0.6V with 2% Accuracy
CMCOT ™ Control Loop Design
Suitable for use in SnPb or Pb-free soldering processes.
Simplified Application Circuit
L
VIN
VIN
CIN
LX
RT5712B
EN
DS5712B-00
April 2015
R1
FB
GND
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
VOUT
COUT
R2
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
1
RT5712B
Functional Pin Description
Pin No.
Pin Name
Pin Function
1
IC
No Internal Connection.
2
EN
Enable Control Input.
3
VIN
Supply Voltage Input. The RT5712B operates from a 2.7V to 5.5V input.
4
LX
Switch Node.
Ground. The exposed pad must be soldered to a large PCB and connected to
GND for maximum thermal dissipation.
5, 7 (Exposed Pad) GND
6
FB
Feedback.
Function Block Diagram
EN
VIN
UVLO
Shut Down
Control
OTP
-
FB
VREF
Error
Amplifier
Ton
Comparator
+
RC
CCOMP
+
-
Logic
Control
LX
VIN
Driver
Current
Limit
Detector
Current
Sense
LX
GND
LX
Operation
The RT5712B is a synchronous low voltage step-down
converter that can support the input voltage range from
2.7V to 5.5V and the output current can be up to 2A.
The RT5712B uses a constant on-time, current mode
architecture. In normal operation, the high side
P-MOSFET is turned on when the switch controller is
set by the comparator and is turned off when the Ton
comparator resets the switch controller.
Low side MOSFET peak current is measured by
internal RSENSE. The error amplifier EA adjusts
COMP voltage by comparing the feedback signal (VFB)
from the output voltage with the internal 0.6V reference.
When the load current increases, it causes a drop in
the feedback voltage relative to the reference, then the
COMP voltage rises to allow higher inductor current to
match the load current.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
www.richtek.com
2
UV Comparator
If the feedback voltage (VFB) is lower than threshold
voltage 0.2V, the UV comparator's output will go high
and the switch controller will turn off the high side
MOSFET. The output under voltage protection is
designed to operate in Hiccup and Latch-off mode.
Enable Comparator
A logic-high enables the converter; a logic-low forces
the IC into shutdown mode.
Soft-Start (SS)
An internal current source charges an internal capacitor
to build the soft-start ramp voltage. The VFB voltage will
track the internal ramp voltage during soft-start interval.
The typical soft-start time is 1.2ms.
is a registered trademark of Richtek Technology Corporation.
DS5712B-00
April 2015
RT5712B
Over Current Protection (OCP)
The RT5712B provides over current protection by
detecting low side MOSFET valley inductor current. If
the sensed valley inductor current is over the current
limit threshold (2.9A typ.), the OCP will be triggered.
When OCP is tripped, the RT5712B will keep the over
current threshold level until the over current condition is
removed.
Thermal Shutdown (OTP)
The device implements an internal thermal shutdown
function when the junction temperature exceeds 150°C.
The thermal shutdown forces the device to stop
switching when the junction temperature exceeds the
thermal shutdown threshold. Once the die temperature
decreases below the hysteresis of 20°C, the device
reinstates the power up sequence.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS5712B-00
April 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
3
RT5712B
Absolute Maximum Ratings
(Note 1)

Supply Input Voltage ---------------------------------------------------------------------------------------- 0.3V to 6.5V

LX Pin Switch Voltage--------------------------------------------------------------------------------------- 0.3V to (VIN + 0.3V)
<20ns ------------------------------------------------------------------------------------------------------------ 4.5V to 7.5V

Power Dissipation, PD @ TA = 25C
WDFN-6L 2x2 ------------------------------------------------------------------------------------------------- 0.833W

Package Thermal Resistance
(Note 2)
WDFN-6L 2x2, JA ------------------------------------------------------------------------------------------- 120C/W
WDFN-6L 2x2, JC ------------------------------------------------------------------------------------------- 7C/W

Lead Temperature (Soldering, 10 sec.) ----------------------------------------------------------------- 260C

Junction Temperature --------------------------------------------------------------------------------------- 40C to 150C

Storage Temperature Range ------------------------------------------------------------------------------ 65C to 150C

ESD Susceptibility
(Note 3)
HBM (Human Body Model) -------------------------------------------------------------------------------- 2kV
Recommended Operating Conditions
(Note 4)

Supply Input Voltage ---------------------------------------------------------------------------------------- 2.7V to 5.5V

Ambient Temperature Range------------------------------------------------------------------------------ 40C to 85C

Junction Temperature Range ----------------------------------------------------------------------------- 40C to 125C
Electrical Characteristics
(VIN = 3.6V, TA = 25C, unless otherwise specified)
Parameter
Symbol
Min
Typ
Max
Unit
2.7
--
5.5
V
0.588
0.6
0.612
V
VFB = 3.3V
--
--
1
A
Active, VFB = 0.63V, Not Switching
--
300
--
Shutdown
--
--
1
Switching Leakage Current
--
--
1
A
Switching Frequency
--
1
--
MHz
Input Voltage
VIN
Feedback Reference Voltage
VREF
Feedback Leakage Current
IFB
DC Bias Current
Test Conditions
A
Switch On Resistance, High
RPMOS
ISW = 0.3A
--
100
--
m
Switch On Resistance, Low
RNMOS
ISW = 0.3A
--
70
--
m
Valley Current Limit
ILIM
2.1
2.9
3.8
A
Under-Voltage Lockout
Threshold
VUVLO
VDD Rising
--
2.25
2.5
VDD Falling
--
2
--
--
150
--
Over-Temperature Threshold
Enable Input
Voltage
www.richtek.com
4
C
Logic-High
VIH
1.5
--
--
Logic-Low
VIL
--
--
0.4
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
V
V
is a registered trademark of Richtek Technology Corporation.
DS5712B-00
April 2015
RT5712B
Parameter
Min
Typ
Max
Unit
--
1.2
--
ms
Minimum Off Time
--
120
--
ns
Output Discharge Switch On
Resistance
--
1.8
--
k
Soft-Start Time
Symbol
Test Conditions
TSS
Note 1. Stresses beyond those listed “Absolute Maximum Ratings” may cause permanent damage to the device. These are
stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the
operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions may affect
device reliability.
Note 2. JA is measured at TA = 25C on a high effective thermal conductivity four-layer test board per JEDEC 51-7. JC is
measured at the exposed pad of the package.
Note 3. Devices are ESD sensitive. Handling precaution recommended.
Note 4. The device is not guaranteed to function outside its operating conditions.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS5712B-00
April 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
5
RT5712B
Typical Application Circuit
L
VIN
2.7V to 5.5V
3
CIN
22µF
VIN
LX
4
CFF*
RT5712B
2
EN
FB
GND
VOUT
R1
6
COUT
5
R2
*CFF : Optional for performance fine-tune
Table 1. Suggested Component Values
VOUT (V)
R1 (k)
R2 (k)
L (H)
COUT (F)
3.3
90
20
1.5
22
1.8
100
50
1.5
22
1.5
100
66.6
1.5
22
1.2
100
100
1.5
22
1.05
100
133
1.5
22
1
100
148
1.5
22
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
www.richtek.com
6
is a registered trademark of Richtek Technology Corporation.
DS5712B-00
April 2015
RT5712B
Typical Operating Characteristics
Efficiency vs. Output Current
Efficiency vs. Output Current
100
100
90
90
80
VIN = 5V, VOUT = 3.3V
70
Efficiency (%)
Efficiency (%)
80
VIN = 3.3V, VOUT = 1.2V
60
50
40
70
40
30
20
20
10
10
0
0.001
0
0.5
1
1.5
VIN = 3.3V, VOUT = 1.2V
50
30
0
VIN = 5V, VOUT = 3.3V
60
2
0.01
10
Output Voltage vs. Output Current
Output Voltage vs. Output Current
1.28
3.40
1.26
3.38
Output Voltage (V)
1.24
1.22
1.20
1.18
1.16
3.36
3.34
3.32
3.30
3.28
1.14
VIN = 3.3V, VOUT = 1.2V
VIN = 5V, VOUT = 3.3V
1.12
3.26
0
0.5
1
1.5
2
0
0.5
Output Current (A)
1
1.5
2
Output Current (A)
Output Voltage vs. Input Voltage
Output Voltage vs. Input Voltage
1.25
3.50
1.24
3.45
1.23
3.40
Output Voltage (V)
Output Voltage (V)
1
Output Current (A)
Output Current (A)
Output Voltage (V)
0.1
1.22
1.21
1.20
1.19
1.18
1.17
3.35
3.30
3.25
3.20
3.15
3.10
1.16
3.05
VIN = 2.5V to 5.5V, V OUT = 1.2V, IOUT = 1A
1.15
VIN = 4.5V to 5.5V, V OUT = 3.3V, IOUT = 1A
3.00
2.5
3
3.5
4
4.5
5
Input Voltage (V)
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS5712B-00
April 2015
5.5
4.5
4.6
4.7
4.8
4.9
5
5.1
5.2
5.3
5.4
5.5
Input Voltage (V)
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
7
RT5712B
Switching Frequency vs. Input Voltage
1.5
0.64
1.4
Switcing Frequency (MHz)
Reference Voltage (V)
Reference Voltage vs Input Voltage
0.65
0.63
0.62
0.61
0.60
0.59
0.58
0.57
0.56
1.3
1.2
1.1
1.0
0.9
0.8
0.7
0.6
IOUT = 0.6A
0.55
IOUT = 0.6A
0.5
2.5
3
3.5
4
4.5
5
5.5
2.5
3
3.5
Input Voltage(V)
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
VEN = 0
-0.1
3
3.5
4
4.5
5
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
VEN = 0
0.0
5.5
-50
-25
0
25
50
75
100
125
Temperature (°C)
Quiescent Current vs. Input Voltage
Quiescent Current vs. Temperature
400
400
380
380
Quiescent Current (μA)
Quiescent Current (μA)
5.5
5.0
Input Voltage (V)
360
340
320
300
280
260
240
220
5
Shutdown Quiescent Current vs. Temperature
Shutdown Quiescent Current (μA)1
Shutdown Quiescent Current (μA)1
1.0
2.5
4.5
Input Voltage (V)
Shutdown Quiescent Current vs. Input Voltage
0.0
4
360
340
320
300
280
260
240
220
VFB = 0.63V, LX No Switch
200
VIN = 5V
200
2.5
3
3.5
4
4.5
5
Input Voltage(V)
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
www.richtek.com
8
5.5
-50
-25
0
25
50
75
100
125
Temperature (°C)
is a registered trademark of Richtek Technology Corporation.
DS5712B-00
April 2015
RT5712B
Inductor Current Limit vs. Temperature
5
4
4
Inductor Current (A)
Inductor Current (A)
Inductor Current Limit vs. Input Voltage
5
3
2
3
2
1
1
VOUT = 1.2V
VOUT = 1.2V
0
0
2.5
3
3.5
4
4.5
5
-50
5.5
-25
0
Input UVLO vs. Temperature
50
75
100
125
Enable Threshold vs. Temperature
2.5
1.4
2.4
Enable Threshold (V) 1
1.2
2.3
Input UVLO (V)
25
Temperature (°C)
Input Voltage (V)
Turn Off
2.2
2.1
2.0
1.9
Turn On
1.8
1.7
1.0
Enable On
0.8
Enable Off
0.6
0.4
0.2
1.6
VIN = 3.3V
VEN = 3.3V
0.0
1.5
-50
-25
0
25
50
75
100
-50
125
0
25
50
75
100
Temperature (°C)
Load Transient Response
Load Transient Response
125
VIN = 3.3V, VOUT = 1.2V, IOUT = 1A to 2A
VIN = 3.3V, VOUT = 1.2V, IOUT = 0A to 1A
VOUT
(50mV/Div)
VOUT
(50mV/Div)
IOUT
IOUT
(1A/Div)
(1A/Div)
Time (100s/Div)
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS5712B-00
-25
Temperature (°C)
April 2015
Time (100s/Div)
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
9
RT5712B
Voltage Ripple
Voltage Ripple
VIN = 5V, VOUT = 3.3V, IOUT = 2A
VIN = 3.3V, VOUT = 1.2V, IOUT = 2A
VOUT
(10mV/Div)
VOUT
(10mV/Div)
VLX
(2V/Div)
VLX
(2V/Div)
Time (500ns/Div)
Time (500ns/Div)
Power On from EN
Power Off from EN
VEN
(5V/Div)
VEN
(5V/Div)
VIN = 3.3V, VOUT = 1.2V, IOUT = 2A
VIN = 3.3V, VOUT = 1.2V, IOUT = 2A
VOUT
(1V/Div)
VOUT
(1V/Div)
IOUT
(2A/Div)
IOUT
(2A/Div)
Time (500s/Div)
Time (10s/Div)
Power On from EN
Power Off from EN
VEN
(5V/Div)
VEN
(5V/Div)
VOUT
(1V/Div)
VOUT
(3V/Div)
VIN = 5V, VOUT = 3.3V, IOUT = 2A
IOUT
(2A/Div)
IOUT
(2A/Div)
Time (500s/Div)
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
www.richtek.com
10
VIN = 5V, VOUT = 3.3V, IOUT = 2A
Time (10s/Div)
is a registered trademark of Richtek Technology Corporation.
DS5712B-00
April 2015
RT5712B
Application Information
The RT5712B is a single-phase step-down converter. It
provides single feedback loop, constant on-time current
voltage at the FB pin, causing PWM pulse width to
increase slowly and in turn reduce the input surge
mode control with fast transient response. An internal
0.6V reference allows the output voltage to be precisely
regulated for low output voltage applications. A fixed
switching frequency (1MHz) oscillator and internal
compensation are integrated to minimize external
component count. Protection features include over
current protection, under voltage protection and over
temperature protection.
current. The internal 0.6V reference takes over the loop
control once the internal ramping-up voltage becomes
higher than 0.6V.
Output Voltage Setting
UVLO Protection
Connect a resistive voltage divider at the FB between
VOUT and GND to adjust the output voltage. The output
voltage is set according to the following equation :
The RT5712B has input Under Voltage Lockout
protection (UVLO). If the input voltage exceeds the
UVLO rising threshold voltage (2.25V typ.), the
converter resets and prepares the PWM for operation.
If the input voltage falls below the UVLO falling
threshold voltage during normal operation, the device
VOUT = VREF   1 R1 
 R2 
where VREF is the feedback reference voltage 0.6V
(typ.).
VOUT
R1
R2
GND
Figure 1. Setting VOUT with a Voltage Divider
Chip Enable and Disable
The EN pin allows for power sequencing between the
controller bias voltage and another voltage rail. The
RT5712B remains in shutdown if the EN pin is lower
than 400mV. When the EN pin rises above the VEN trip
point, the RT5712B begins a new initialization and
soft-start cycle.
Internal Soft-Start
The RT5712B provides an internal soft-start function to
prevent large inrush current and output voltage
overshoot when the converter starts up. The soft-start
(SS) automatically begins once the chip is enabled.
During soft-start, the internal soft-start capacitor
becomes charged and generates a linear ramping up
voltage across the capacitor. This voltage clamps the
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
April 2015
The RT5712BL provide Over Voltage Protection
function when output voltage over 120%. The IC will be
into Latch-off mode.
will stop switching. The UVLO rising and falling
threshold voltage has a hysteresis to prevent
noise-caused reset.
Inductor Selection
The switching frequency (on-time) and operating point
FB
DS5712B-00
Over Voltage Protection (OVP)
(% ripple or LIR) determine the inductor value as
shown below :
L=
fSW
VOUT   VIN  VOUT 
 LIR  ILOAD(MAX)  VIN
where LIR is the ratio of the peak-to-peak ripple current
to the average inductor current.
Find a low loss inductor having the lowest possible DC
resistance that fits in the allotted dimensions. The core
must be large enough not to saturate at the peak
inductor current (IPEAK) :
IPEAK = ILOAD(MAX) +  LIR  ILOAD(MAX) 
 2

The calculation above serves as a general reference.
To further improve transient response, the output
inductor can be further reduced. This relation should be
considered along with the selection of the output
capacitor.
Inductor saturation current should be chosen over IC’s
current limit.
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
11
RT5712B
Input Capacitor Selection
High quality ceramic input decoupling capacitor, such
as X5R or X7R, with values greater than 10F are
recommended for the input capacitor. The X5R and
X7R ceramic capacitors are usually selected for power
regulator capacitors because the dielectric material has
less capacitance variation and more temperature
stability.
Voltage rating and current rating are the key
parameters when selecting an input capacitor.
Generally, selecting an input capacitor with voltage
rating 1.5 times greater than the maximum input
voltage is a conservatively safe design.
The input capacitor is used to supply the input RMS
current, which can be approximately calculated using
the following equation :
IIN_RMS = ILOAD 
VOUT
VIN
V
 1 OUT 
VIN 

The next step is selecting a proper capacitor for RMS
current rating. One good design uses more than one
capacitor with low equivalent series resistance (ESR) in
parallel to form a capacitor bank.
The input capacitance value determines the input ripple
voltage of the regulator. The input voltage ripple can be
approximately calculated using the following equation :
VIN =
IOUT(MAX) VOUT  VOUT 

 1
CIN  fSW
VIN 
VIN 
Output Capacitor Selection
The output capacitor and the inductor form a low pass
filter in the Buck topology. In steady state condition, the
ripple current flowing into/out of the capacitor results in
ripple voltage. The output voltage ripple (VP-P) can be
calculated by the following equation :
1

VP_P = LIR  ILOAD(MAX)   ESR +

8

C

f
OUT
SW


For a given output voltage sag specification, the ESR
value can be determined.
Another parameter that has influence on the output
voltage sag is the equivalent series inductance (ESL).
The rapid change in load current results in di/dt during
transient. Therefore, the ESL contributes to part of the
voltage sag. Using a capacitor with low ESL can obtain
better transient performance. Generally, using several
capacitors connected in parallel can have better
transient performance than using a single capacitor for
the same total ESR.
Thermal Considerations
For continuous operation, do not exceed absolute
maximum junction temperature. The maximum power
dissipation depends on the thermal resistance of the IC
package, PCB layout, rate of surrounding airflow, and
difference between junction and ambient temperature.
The maximum power dissipation can be calculated by
the following formula :
PD(MAX) = (TJ(MAX)  TA) / JA
where TJ(MAX) is the maximum junction temperature,
TA is the ambient temperature, and JA is the junction to
ambient thermal resistance.
For recommended operating condition specifications,
the maximum junction temperature is 125C. The
junction to ambient thermal resistance, JA, is layout
dependent. For WDFN-6L 2x2 package, the thermal
resistance, JA, is 120C/W on a standard four-layer
thermal test board. The maximum power dissipation at
TA = 25C can be calculated by the following formula :
PD(MAX) = (125C  25C) / (120C/W) = 0.833W for
WDFN-6L 2x2 package
The maximum power dissipation depends on the
operating ambient temperature for fixed TJ(MAX) and
thermal resistance, JA. The derating curve in Figure 2
allows the designer to see the effect of rising ambient
temperature on the maximum power dissipation.
When load transient occurs, the output capacitor
supplies the load current before the controller can
respond. Therefore, the ESR will dominate the output
voltage sag during load transient. The output voltage
undershoot (VSAG) can be calculated by the following
equation :
VSAG = ILOAD  ESR
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
www.richtek.com
12
is a registered trademark of Richtek Technology Corporation.
DS5712B-00
April 2015
RT5712B
Maximum Power Dissipation (W)1
1.0
Four-Layer PCB
0.8
0.6
0.4
0.2
0.0
0
25
50
75
100
125
Ambient Temperature (°C)
Figure 2. Derating Curve of Maximum Power
Dissipation
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
DS5712B-00
April 2015
is a registered trademark of Richtek Technology Corporation.
www.richtek.com
13
RT5712B
Outline Dimension
Symbol
Dimensions In Millimeters
Dimensions In Inches
Min
Max
Min
Max
A
0.700
0.800
0.028
0.031
A1
0.000
0.050
0.000
0.002
A3
0.175
0.250
0.007
0.010
b
0.200
0.350
0.008
0.014
D
1.950
2.050
0.077
0.081
D2
1.000
1.450
0.039
0.057
E
1.950
2.050
0.077
0.081
E2
0.500
0.850
0.020
0.033
e
L
0.650
0.300
0.026
0.400
0.012
0.016
W-Type 6L DFN 2x2 Package
Richtek Technology Corporation
14F, No. 8, Tai Yuen 1st Street, Chupei City
Hsinchu, Taiwan, R.O.C.
Tel: (8863)5526789
Richtek products are sold by description only. Richtek reserves the right to change the circuitry and/or specifications without notice at any time. Customers should
obtain the latest relevant information and data sheets before placing orders and should verify that such information is current and complete. Richtek cannot assume
responsibility for use of any circuitry other than circuitry entirely embodied in a Richtek product. Information furnished by Richtek is believed to be accurate and
reliable. However, no responsibility is assumed by Richtek or its subsidiaries for its use; nor for any infringements of patents or other rights of third parties which may
result from its use. No license is granted by implication or otherwise under any patent or patent rights of Richtek or its subsidiaries.
Copyright © 2014 Richtek Technology Corporation. All rights reserved.
www.richtek.com
14
is a registered trademark of Richtek Technology Corporation.
DS5712B-00
April 2015